Sunday, 5 June 2011

The Spearhead Dinghy

In March I mentioned a boat, a dinghy called Spearhead, that I helped design many years ago. Two of them have been sitting on my back yard for the last decade, sailing very occasionally. 
One has now gone to a new home. After a couple of weeks of refurbishment, we got it sailing. Not as dramatic an event as we might have liked because there was no wind but it did ghost along. We now have the other one on Ebay.
So, as a digression from my normal obsessions, here is a little of the history of the boat and what makes it special.
The project started out when my father was measurer for the 5o5 world championships. (For those of you who don’t know about such things, that is a class of sailing dinghy, five point O five meters long). The measurer has to make sure that all the boats are the same, so inevitably by the end of the week we knew all the ways that people tried to cheat in order to make the boats go a bit faster.
Flatter and longer with a deeper point to the bow seemed to be the main things and make it lighter if you can. In a 5o5 all those things are illegal, but what about making a new class?
Making a boat lighter, calls for some engineering innovation. In a sailing boat, there are forces at work that will break the hull. Start with the mast, it’s not difficult to see that this produces a downward pressure on the bottom of the boat which would make a hole and sink it. Normally the boat has extra thickening to resist this, but that adds weight. When the wind blows on the sail it pushes the mast sideways, so we resist that with side stays, but they pull the sides of the boat inwards. Also, if you do that maths and triangulate the forces, you will see that tightening the side stays also pulls the mast downwards even harder. Actually, it is all in three dimensions, and some of the force goes through the forestay and that too can deform the hull.

The solution to all this came to me lying in bed one night. A triangular aluminium tube frame fitted under the foredeck from which the mast could be suspended. The stays are attached to the frame so that all the rigging forces are contained. The whole thing weights about 10 pounds and because aluminium is light and the arrangement is very efficient, it allowed us to reduce the weight of the hull by about 80 pounds compared to a similar sized racing dinghy.
The rest of the boat was also packed with innovations. The deck was made in one big moulding, using epoxy resin for the fibreglass because that is stronger weight for weight than the more traditional polyester resins that were traditional at the time. We had a theory that by taking a lot of the stress out of the shell and into the frame system the boat would last a lot longer. Thirty odd years later I can say that we were right, not many wood or fibreglass dinghies last as well.
We created the shape by working in a big shed with a trapeze wire hung from the roof. We spend hours sliding in and out, simulating the movements that would be required when sailing. We gradually built up the shape with car body filler, sandpapering and polishing until we had something we could move around with ease. From that plug, we made a mould so that the whole deck of the boat could be cast as a single entity and dropped onto the hull.
The result, as you can see from the pictures, looks more like a modern racing car than a traditional boat. Of course, they make the cars out of carbon fibre, which would have been nice, and even lighter but we weren’t millionaires.
Why did we call it Spearhead?
 Because the shape of the wetted area when it floats looks like a spearhead, well, it would if you were swimming underneath. There are other clever things about the design, on which I could bore for England, but I’ll save those for anyone who asks.
I’m getting too arthritic for cold-water sports, and all the team that built the boats are dead now, apart from me. This is the last one left in the family after free cycle locally found a home for my old boat, so I’m writing this in the hope that we can find a good home for a piece of history that, as the pictures show, is also a load of fun


  1. Awesome! It's a pleasure to hear someone working from first principles, especially when dinghy design seems to have degenerated into copy the last most successful Int14, Nat12, or some other silly skiff.

  2. Hi,
    Thanks you for your great design, I own one of the spearhead, its numbered 17 I think...
    I like it very much

  3. Hi, I purchased from Tenspar a space frame ala spearhead and had it moulded in situ into merlin rocket 3231. A NSM Fibre glass design built by Rowsell. I sailed her for around 12 years before scrapping the hull. She had an unfindable leak that allowed water into the egg box hull and ruined her abilities. The space frame lived up to its design specifictions and allowed the boat to sail well. Sadly lack of time on the water meant I nevr fully got to grips with her capability. I still have the mast in the back garden. Todays technologie would replace all of the frame and mast and boom with carbon fibre tubes and struts saving just a huge amount of weight. I am contemplating resurrecting the installation sometime in 2012, using an aerofoil section.

  4. Thanks for the post. I remember my dad putting the frames into some other classes, that was probably one of them. I agree that with modern materials you could make it even lighter. Looking back now it seems to me that the shape of the Spearhead bears a lot of resemblance to Formula 1 cars that are mostly carbon fibre. Of course they spend a lot of money. Back then we were just trying to get rid of the weight of the hog, kingpost and thwarts that were common in dinghys, and necessary to stop the rigging forces wrecking the boat. On some of the early 470s you couldn't get the plate down if you had the rig too tight. We saved about 80 pounds compared to a 5o5.

  5. Hi Spearhead number 16 still loved and sailing well @ BVSC. Any info about boat is always appreciated and never boring. Thanks

  6. Can this boat be built from plywood and plans?

    1. Thank you Anonymous for the question. As I can't reply to you directly I will say something here.
      We designed the Spearhead in glass fibre but the principles could be applied to any other form of construction. Essentially the concept is about making a frame structure that takes the forces in the rigging so that it is possible to design a hull that can be as light as possible. The purpose of the hull is to keep out the water and provide something to sit on.
      It is of course more complicated than that, there are other forces that have to be considered; those from the centre plate or keel, those from the rudder and those caused by the crew moving about. Building in glass fibre allowed us to deal with those hull forces by engineering the shape of the hull and by including additional elements in the glass lay up, like unidirectional glass in key places and using epoxy resin. We also used a vacuum pressure to keep the e wanted it. Current technologies, like using carbon fibre and kevlar would probably be even better.
      It would be possible to do all this in plywood, but that would require some design work to think through the forces involved and calculate how much wood was needed in each critical place. It would be fun to do, but we didn't actually do it.
      If I were doing it now I would be tempted to try to design a sort of internal chassis to take the rudder and centre plate forces so that the outer skin could be kept as light as possible.
      We did, at various times put the space frame rig into other boats, but that approach was limited by the design rules in those boats. I know we did one for an International 14, for instance. Although the all up weight of these classes is usually fixed the space frame allows you to build a stiffer boat and to have more freedom to keep the weight out of the ends.
      So without rambling any more, it would be possible to build a boat like this in wood, but it would have to be designed. When we stopped making them we deliberately allowed the patent on the rigging system to lapse, so there is nothing to stop anyone having a go.

  7. Hi Rod, I was delighted to find your piece on the Spearhead that for obvious reasons was quite popular in South Africa. I had one as a student in the 70s and later acquired one that was abandoned in a yard when the previous owners left. I sailed it on the Albasini dam. My brother also owns one and we had great fun in the Around the Island race on the Vaal dam. Fantastic boat!

    1. Couldn't resist adding the comment, only problem is it's not the same boat. We had never heard of the South African version at the time we designed the Spearhead in the UK. Back then we weren't talking to South Africa as I recall.

  8. The South African boat is different. Cold molded. Looks like a bigger Int 14 (plumb bow)
    Probably 20 years earlier.